Reducing TGP Reservoir Deposition By Dredging Coarse Sediment

A scheme of dredging coarse sediment (CS) for reservoir desiltation from a fixed dredging basin (FB) in the fluctuating backwater of river shaping reservoir is studied herein this paper. The prototype data indicate an obvious fact that grain size sorting along the reservoir from upstream to the deep pools, and sediment deposited in the fluctuation backwater reaches (FBW) is few in quantity but it consistes mostly of coarse sand, which will become the main frame of future deposition delta. This scheme focused on removing CS from specific locations where mostly only CS deposited in the shallow reaches of the FBW by the dredger, to reduce the deposition of reservoir, and lessen the ultimate talweg and backwater.The efficiency of reducing reservoir deposition is high by the method of dredging CS. According to the theoretic analysis, the setting FB in the FBW can be functioned as a sand collector for removing CS constantly and it will be able to intercept most CS into the deep pools, on the basis that deposited ratio of CS is more than the fine sand because of the positive relativity between deposited ratio and settling velocity. In addition, the sedimentation equilibrium gradient slow down as the sand transporting to downstream is fined because of the interception of CS. After dredging, the whole reservoir is in desiltation under the interaction between the changes of dynamic condition and fluvial evolution. The author reviewed the increasing capacity of dredging that promoted by fast development and commercial needs of dredgers and large scale dredging projects, and the market of river-sand along the Yangtze in particular in recent decades. It implies that reservoir dredging is not as expensive and low efficient as it was previously believed and large scale dredging for reservoir desiltation seems to be worthy of consideration.Applying a well-applied model of unsteady flow and sediment transport, the author studied many cases with respect to different flow and sediment conditions for TGP. Computation reveals that the scale of CS dredged is 32-38×10~8m~3 and 18-26×10~8m~3 for 60s and 90s condition respectively, and 100years accumulated deposition after dredging is to be decreased to 140×10~8m~3 and 110×10~8m~3. The efficiency of desiltation by dredging is as high as 95%, and interception ratio of CS is 50%-70% which result in the 25% decreasing of slope. The great reducing of deposition of Chongqing reach can restrain the deposit extension and backwater elevation. It seems to the author that this scheme for TGP is a sustainable way for that can keep the reservoir lastly in low level of deposition and protect its flood control capacity and tail waterway safety, which is important of TGP.This scheme is also worthwhile as a reference for desiltation of others like the TGP of a river shaped reservoir.